Upgrade panel-meter range and

2011-11-14 10:52:16来源: 互联网
d also offers full control over the LED display. A microcontroller can easily use this interface to read the ADC result and control what is shown on the display.

Circuitry for doubling the input range is as simple as a resistor divider that cuts the signal in half, allowing the IC's input-voltage range to remain ±200mV. Instead of VIN, the ADC now measures VIN/2. You can easily compensate this factor of two by shifting the result one bit to the left (digital multiplication by 2).

The ADC resolution is 20 bits, which is more than enough to support 4¾-digit precision. (Normally, the IC obtains 4½ digit precision using only 16 of the 20 available bits.) Figure 1shows how the data is rearranged. The 17th bit is used to fill the gap caused by the multiplication by 2.

The resistor divider adds gain error, so you should choose resistors whose accuracy meets the requirements of your application.


Figure 1. The range of this panel-meter circuit is doubled by scaling the input signal, and its precision is doubled by using more of the available bits.

A 7-segment display driver internal to the panel-meter IC drives a common-cathode LED display, using a multiplexing scheme to drive one digit at a time. Unfortunately the driver is optimized for driving 4½-digit displays. That means that the first (most significant) position can only show a "half digit" of +1 or -1 (Figure 2).


Figure 2. Seven-segment displays include half digits (left) and full "1" digits (right).

Signals via the SPI interface can switch all segments in the display on and off individually, except for those in the first digit. Segments B and C (which form "1") can only be switched on or off together. The same is true for segments A, F, and D (which form "+"). Segment G (which forms "-") is on when segments A, F, and D are off, and vice versa. To upgrade this display to 4¾ digits, you must ensure that the first digit position can show "1," "2," "3," or a blank (Figure 3). From the figure it is clear that segment F is never used, so you need only control six of the segments.


Figure 3. In the four possible states of a ¾-digit display, segment F is always off.

If the ½-digit display is replaced by a full "1-digit" display, and control lines for the F and G segments are swapped, all segments but F are lit when the "1" and "+" signs are activated (Figure 4). The E segment is not used in the ½-digit display, and if left connected is on by default. Adding switches in the form of low-cost NPN transistors enables a microcontroller to turn each segment on and off via a few GPIO pins, producing a display of blank, "1," "2," or "3." The complete circuit is shown in Figure 5.


Figure 4. To convert a ½-digit display to a full "1-digit" display, swap the control lines to the F and G segments.


Figure 5. Complete 4¾-digit panel-meter circuit.

The end of conversion (EOC\) signal connects to an external interrupt on the microcontroller. When a new value appears, the microcontroller reads it through the SPI port and multiplies it by 2. It then determines which segments to illuminate (switch on) by using the SPI port to set correct values in the display registers. External transistor switches are then set via the GPIO pins to create the desired character.

In front of the first digit you can use a rectangular LED as a minus sign, controlled by the signal of an unused decimal point. Any microcontroller with an external interrupt pin, an SPI interface, and enough GPIOs is acceptable for this application. The test setup includes aMAX1499 panel-driver IC on the display board, controlled by a low-power RISC microcontroller (MAXQ610) on its own evaluation board (Figure 6). Firmware is available for download.

关键字:range

编辑:神话 引用地址:http://www.eeworld.com.cn/mndz/2011/1114/article_13120.html
本网站转载的所有的文章、图片、音频视频文件等资料的版权归版权所有人所有,本站采用的非本站原创文章及图片等内容无法一一联系确认版权者。如果本网所选内容的文章作者及编辑认为其作品不宜公开自由传播,或不应无偿使用,请及时通过电子邮件或电话通知我们,以迅速采取适当措施,避免给双方造成不必要的经济损失。
论坛活动 E手掌握
微信扫一扫加关注
论坛活动 E手掌握
芯片资讯 锐利解读
微信扫一扫加关注
芯片资讯 锐利解读
推荐阅读
全部
range

小广播

独家专题更多

富士通铁电随机存储器FRAM主题展馆
富士通铁电随机存储器FRAM主题展馆
馆内包含了 纵览FRAM、独立FRAM存储器专区、FRAM内置LSI专区三大部分内容。 
走,跟Molex一起去看《中国电子消费品趋势》!
走,跟Molex一起去看《中国电子消费品趋势》!
 
带你走进LED王国——Microchip LED应用专题
带你走进LED王国——Microchip LED应用专题
 
电子工程世界版权所有 京ICP证060456号 京ICP备10001474号 电信业务审批[2006]字第258号函 京公海网安备110108001534 Copyright © 2005-2016 EEWORLD.com.cn, Inc. All rights reserved